Chemical Forums

Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: simonwei on September 11, 2010, 10:09:29 AM

Title: A GPC question
Post by: simonwei on September 11, 2010, 10:09:29 AM: Hi, Dear all:

I was asked to make a new batch of PMMA-based polymer published before and characterize the molecular weight by GPC. The eluent used in the paper is DMF with 0.01 M lithium nitrate. We tried THF first because we have done the calibration for THF eluent, the polymer is well solube in THF, and we didn't want to go through the hassel of switching to DMF. I used a typical concentration: 1 mg polymer / 1 ml THF, but could not get any signal. What could be the possible reason?
Besides, what is the purpose of lithium nitrate in the eluent of DMF?
Thank you very much for your help.

simonwei
Title: Re: A GPC question
Post by: Train on September 12, 2010, 04:02:10 PM: What kind of detector?
Title: Re: A GPC question
Post by: simonwei on September 13, 2010, 12:57:33 AM: The GPC is coupled with a refractive index detector and a differential pressure detector.
Title: Re: A GPC question
Post by: PolymerKnowHow on September 16, 2010, 02:33:56 PM: The lithium nitrate is there to reduce polymer-packing interactions and to promote fractionation based on hydrodynamic volume. Most people use lithium bromide, but I guess the lithium nitrate will accomplish the same thing.

What exactly does PMMA-based mean? Would you consider your polymer polar?
Title: Re: A GPC question
Post by: simonwei on September 19, 2010, 06:23:53 PM: Thanks, PolymerKnowHow, for your help. Could you please elaborate on how the electrolyte reduce the polymer-packing (polystyrene gel in my case) interaction and promote fractionation? Or do you suggest any references to read?
My polymer is a coumarin side chain on PMMA backbone. I don't think this polymer is polar.

Quote from: PolymerKnowHow on September 16, 2010, 02:33:56 PM
The lithium nitrate is there to reduce polymer-packing interactions and to promote fractionation based on hydrodynamic volume. Most people use lithium bromide, but I guess the lithium nitrate will accomplish the same thing.

What exactly does PMMA-based mean? Would you consider your polymer polar?
Title: Re: A GPC question
Post by: PolymerKnowHow on September 20, 2010, 02:18:58 PM: Polar macromonomers commonly aggregate together in DMF. This will cause for earlier elution times and subsequently very high molecular weights.

But I have seen the opposite with PVDF; where the polymer adsorbs to the packing material and elutes after the solvent peak.

On that note, can you confirm that the polymer never elutes? It does not elute early? It does not elute late?